Enamelware



Patented July 21, 1942 ENAMELWARE William J.

Baldwin, Ingram, Pa., assignor to Enamelers Guild, Inc., a corporationof Pennsylvania No Drawing. Application April 12, 1939, Serial No.267,523

2 Claims.

The invention relates to enamel-ware; that is to say, articles of metal(ordinarily of steel) bearing superficial coatings of glass. It isdirected to the production of an article of enamel-ware of uniquesurface configuration and decorative appearance, and it involves noveltyboth in'the composition of the frit, in the method of maturing thecoating, and in the product.

In the prevailing practice of enamel-ware making a suitable batch ofmaterial is smelted to glass, the glass is cooled, solidified, andcrushed. The crushed glass, called frit, is worked with clay and Waterto a paste called slip, and the slip is applied to the metal surface,typically by dipping. The coating so spread upon the surface of thearticle is dried to hardness, and then the coated article is heated to atemperature at which the glass melts again, takes up the clay as amodifying component, and forms more intimate union with the underlyingmetal. On cooling, the melted glass hardens to an integral and adherentcoating. A second and a third coating may similarly be developed uponthe first.

In Letters Patent of the United States, No. 2,043,559, granted June 9,1936, on the application of Jacob E. Rosenberg, a refinement in themethod of forming the initial coating upon enamel-ware articles isdescribed that involves the addition, to the batch of glass-formingmaterials, of the antimonate of an alkali or of an alkaline-earth metal(typically sodium antimonate) I have been employed by the O. HommelCompany, the assignee and present owner of the said Rosenberg patent,and have been engaged in further investigation along lines laid down bythe said Rosenberg patent.

I have found it convenient, instead of introducing sodium antimonate assuch with the other materials that go to make up the batch, to introduceantimony trioXide ($10406) together with nitrate of soda (NaNOs), withthe consequence that in the smelting operation sodium antimonate formsitself by spontaneous reaction.

My present invention proceeds from the discovery that, in a batch thatincludes a fluoride, if the antimony trioxide addition be in excessquantity, so that there remains in the frit and in the coating that iseventually melted upon the metal surface a quantity of antimony trioxide(the figures will be given below) that has not by oxidation been broughtto the pentavalent form, and if the ultimate firing (that is to say, themelting of the coating of dried and hardened slip upon the metal) becarried out in an oxidizing atmosphere, an unexpected result will ensue,that gives to the finished article a unique and highly decorativefeature of appearance. A skin apparently forms upon the air interface ofthe molten layer of coating, and in cooling and solidification of thelayer the underlying body of glass (it would seem) contracts slightly,with the effect of causing the skin to wrinkle and the wrinkles toorganize themselves into groups radiating from centres that seem to bedetermined by chance. The minutely waved surface, in which order andchance are combined, gives to the finished article a surfaceconfiguration that, under the light of day, modifies the monotony, andimposes upon the otherwise plain and colored surface a pattern thatclosely resembles the surface pattern of galvanized iron. This surfacepattern I shall, for the sake of brevity, characterize the crinklefinish.

The typical formula for the batch, upon which my experiments were based,wasv Parts Dehydrated borax (Na2B4Oi) 15.4 Soda ash (NazCOs) 6. 5 Silica(S102) 23.8 Feldspar (NaK.OAl2Os.6SiOz) 33.2 Cryolite (NasAlFe) 10.5Fluorspar (CaFz) 4. 3 Antimony trioxide (Sb40s) 8. 5 Nitrate of soda(NaNOs) 3. 2

In the smelting, while there is a small loss in total weight, there isno loss in the weights of the cryolite and the antimony trioxideconstituents; and it will be found that in the frit these twoconstituents will continue-cryolite, substantially 10.5%, and antimonytrioxide, substantially 8.5%-the antimony trioxide reacted, as alreadyexplained, to sodium antimonate.

In normal use of the formula, when the batch is compounded and all theingredients are smelted together, the resulting enamel-ware presentsaplain and uniform appearance of surface.

The experiment that led to the discovery consisted in compounding thebatch according to the formula, including the antimony trioxide, butomitting the nitrate of soda, and in adding the nitrate of soda onlyafter the smelting of the other ingredients had been accomplished. Uponthe procedure so modified the unique and wholly unexpected appearance ofthe product, characterized above, ensued. Recognizing its value, Iproceeded to investigate and determine the limiting conditions of itsoccurrence.

If, in place of antimony trioxide and nitrate of soda, sodium antimonate(NaSbOs) be included in or added to the batch (the antimony assuming inthe melt its pentavalent condition-Sb2O5), the crinkle finish will notappear.

If the formula be modified, either by increase in the antimony trioxidecontent or reduction in the nitrate of soda content, until the ratio ofantimony trioxide to nitrate appreciably exceeds 4:1 (procedure beingnormal and all the constituents being smelted together), the crinklefinish will appear in the product. Thus, if the antimony trioxidecontent be held at 8.5 parts and the nitrate of soda content begradually reduced in successive runs, it will be found that the crinklefinish will appear when the nitrate content has fallen below 2 parts,and will be yet more pronounced at 1 part, 0.50 part, etc. And, again,if the nitrate content be held at 2.5 parts and the antimony trioxidecontent be gradually increased, the crinkle finish will appear when-thetrioxide content reaches 11 parts, and will be yet more pronounced asthe trioxide content rises to higher figures.

In any case, I have found that, in order to gain a satisfactory crinklefinish, fluoride being present in quantity given in the formula, theantimony trioxide content should amount to at least 6% of the batch. Ifnitrate of soda be included in the batch, the antimony trioxide contentshould be increased, and should in any case exceed the nitrate of sodacontent in aratio of substantially 8.5:2.

If the cryolite content of the batch is reduced from the typical 10.5parts to as little as 6 parts, the crinkle finish appears in lesspronounced degree. If both the antimony trioxide content and thecryolite content be reducedthe antimony trioxide content to 6 and thecryolite content to 8 (the sodium nitrate being absent)-- the crinklefinish tends to disappear.

If coloring material of a reducing nature be added, the crinkle finishis correspondingly lost. If th article be fired in a non-oxidizingatmosphere, the crinkle finish will not develop. All of which indicatesthe formation in the firing of the article, perhaps by oxidation, of askin at the air interface, and that it is the skin so formed that, asthe body of the coating shrinks in cooling, is thrown into the minutewrinkles that characterize the crinkle finish. My observations lead meto believe that the crinkle finish develops at the very beginning of thecooling of the article, and that sudden cooling through a widetemperature range is not necessary to achieve the result.

These investigations of mine lead to the conclusion that, whereas theresults that Rosenberg has inview in Letters Patent No. 2,043,559 aregained by the reaction of antimonate, present in the glass when thecoating of frit is fired upon the metal article, and Whereas thepresence of antimonate may be insured by including antimony trioxide inthe batch in association with an oxidizing agent, the crinkle finishthat I have discovered is achieved when and only when the batch includesa fluoride and when antimony trioxide in excess is present in the batchand persists in trivalent state in the frit.

I claim as my invention:

1. A frit for enamel-ware adapted to afford in the finished article asurface of waved and ornamental appearance; such frit formed of usualglass-forming ingredients including a; cryolite content of substantially10.5%, in which is present also an unreacted antimony trioxide contentof at least 6%.

2. A batch for enamel ware adapted to afford in the finished article asurface of waved and ornamental appearance, such batch consisting ofusual glass-forming ingredients including a cryolite content exceeding8%, together with antimony trioxide and nitrate of soda, the antimonytrioxide content amounting to at least 6% of the whole and the antimonytrioxide content exceeding the nitrate of soda content more thanfourfold.

WILLIAM J. BALDWIN.

